The YJ and TJ Wrangler manual transmission and
clutch systems are similar and easy to identify. By the late YJ Wrangler era (1993-95), the transmission and
clutch platform for YJ models took an identical form to the TJ Wrangler models that followed in 1997. The
clutch linkage type, clutch and release bearing layouts and even the specific manual transmission offerings
were established during the YJ Wrangler years. Similarly, 3-speed automatic transmissions that were popular
during the 1987-95 years carried forward into the TJ Wrangler models.

Use of hydraulic clutch linkage began in the AMC CJ model era, first
with GM transplant 2.5L ‘Iron Duke’ engines and later with the AMC engines. Hydraulic linkage extended to the
AMC 2.5L four and, eventually, even 4.2L six-cylinder models. These CJ systems relied upon a conventional
clutch linkage layout, using a traditional release arm with an externally mounted slave cylinder. A master
cylinder at the firewall provided the hydraulic force. A pushrod link from the clutch pedal delivered
pressure to the master cylinder apply piston.

The pedal linkage and master cylinder survive from the CJ through the
YJ and TJ Wrangler era. The linkage at the clutch and clutch housing, however, changed abruptly in 1987 with
the introduction of the YJ Wrangler model. Common to that era, an internal clutch release bearing/slave
cylinder replaced the conventional slave cylinder, release arm and release bearing.

In concept, the compact slave/release bearing design eliminates all
mechanical arms and links, relying upon the clutch release bearing and its built-in slave cylinder to deliver
release force. The system requires only a master cylinder, hydraulic lines and the slave/bearing. This
assembly is “self-adjusting” and requires no more attention than topping off or periodically changing the
DOT-3 brake fluid in the hydraulic system.

In theory, the design served well. Its downfall was two-fold: 1) if
the unit leaked or began to fail in service, the fluid would saturate the clutch disc and cause failure of
the clutch assembly and 2) a failed release bearing/slave cylinder required the same labor as a conventional
clutch release bearing to replace. In the case of a Jeep 4x4 with a transfer case and two drivelines, the
prospect of replacing the slave/bearing assembly duplicated the significant labor involved with a clutch
replacement.

By contrast, the 1993 introduction of an external slave cylinder once
again restored the Jeep clutch linkage system to a conventional design. A pivot-type release arm and
familiar, fully-mechanical release bearing returned. The slave cylinder moved outside the clutch housing and
became an easily accessed and serviced unit. In fact, the entire hydraulic linkage system can be readily
replaced without disturbing the transfer case, drivelines, transmission or clutch. In the field, you could
perform such repairs with minimal tools and resources.

Internal versus External Clutch Slave
Cylinders

1987-92 YJ Wranglers use an internal clutch release
bearing with a built-in slave cylinder. Whether the model has the “Peugeot” BA 10/5, AX-4, AX-5 or AX-15
transmission, the slave cylinder is an integral part of the release bearing assembly and fits within the
enclosed clutch housing assembly. Beginning in 1993, the YJ Wrangler clutch linkage switches to the “external
slave cylinder” design, a proven and readily serviceable system.

The example for this article is a 1987 YJ Wrangler, its original
engine was a throttle-body injection 2.5L four. This package came with an AX-5 light duty five-speed
transmission. In the process of building a vehicle to meet more challenging backcountry environments,
oversized tires, family-packed excursions, highway demands, a hardtop and the prospect of toting a cargo
trailer, I replace the original 2.5L four with the popular 4.0L inline EFI/MPI six. A more rugged YJ/XJ/TJ
type Aisin AX-15 transmission replaces the original AX-5. An additional upgrade is the use of the external
slave cylinder package found on later YJs and the TJ Wrangler models.

Illus. 1: During transmission removal, the older AX-5 release
bearing/slave cylinder is visible. This system will not be used upon reassembly. Within this chapter, you
will find the step-by-step rebuild of the AX-15 transmission and installation/upgrade of the front bearing
retainer. This will accommodate the external slave cylinder used with a 1993-up YJ and the TJ clutch
housings.

Illus. 2: This is the ’93-up YJ and TJ Wrangler clutch housing with a
mechanical release arm and a conventional clutch release bearing. These components and an external slave
cylinder will eliminate use of the internal slave/bearing assembly. The transmission requires a new iron
front bearing retainer to accommodate the release bearing design. (See new bearing retainer in the assembly
of the AX-15 transmission.)

Illus. 3: This is the external slave cylinder clutch housing found on
’93 up 4.0L engines. The transmission pattern is an AX-15 or NV3550 type. The slave cylinder mounts outboard
of the clutch housing, easily accessible and simple to remove and replace. Note the pushrod—this part engages
the cross-type mechanical release arm at a point opposite the pivot stud.

Note—AX-15 and NV3550 front face patterns are nearly identical. The
NV3550 clutch housing will interchange with the AX-15 clutch housing. When mating the AX-15 housing to the
later NV3550 transmission, there is a need to die-grind a small area of interfering material from the AX-15
housing. This common pattern increases the clutch housing options for installing either of these
transmissions behind an inline 4.2L or 4.0L Jeep six. Similarly, the AX-15 or NV3550 can even mate to a Jeep
2.5L four using 1990s Dodge Dakota 2.5L four-cylinder components. The Dakota uses an AX-15 for several
years.

Manual Transmissions Used in
Jeep YJ and TJ Wranglers

At the time Chrysler Corporation purchased AMC/Jeep, the YJ Wrangler
was already in production. The partnership of AMC and Renault had produced a variety of innovative products,
and parts fromEurope found their way into the Jeep Corporation products. Among the obvious components was
the introduction of the Peugeot BA 10/5 manual transmission found in 4.2L six-cylinder
YJs.

The BA 10/5 is actually a reasonably robust unit and boasts a 22%
fifth-gear overdrive. Classified as a light-duty transmission, engineered for a vehicle in the YJ Wrangler’s
weight and load class, the Peugeot transmission has provided good service and even proved a candidate for
lighter output V-8 engine transplants.

Today, the shortfalls of this unit are parts availability and the
likelihood that existing Peugeot transmissions have acquired substantial miles and wear. As a result, the BA
10/5 is often discarded, replaced by the more common AX-15 or later NV3550 transmission. Such swaps entail
clutch housing and clutch linkage changes, making this prospect a full-on factory parts “swap” before the
project ends. The BA 10/5 does not appear in YJ Wrangler models after 1989. During 1989 production, the Aisin
AX-15 came on line.

At the outset of YJ Wrangler production, four-cylinder models
received the Aisin AX-4/5 transmissions. (AX-4s were modest four-speed units with 4th gear overdrive and quickly gave way to
the AX-5 inU.S. models.)
The AX-5 with its 15% 5th gear overdrive survives from the earliest YJ models through the 2002 TJ Wranglers
with four-cylinder engines. Built inJapan, the Aisin “AX” transmissions are actually quite durable. They readily match the
performance demands of their assigned YJ and TJ Wrangler models.

The Aisin AX-15 has served behind the 4.2L and 4.0L six-cylinder
engines in YJ Wranglers and the 4.0L six in XJ Cherokee and TJ Wrangler models. The AX-15 would fit the
traditional “light-duty truck” transmission category. As you will discover in the AX-15 rebuilding process,
this is a very rugged transmission and should hold up well even with horsepower in the 250-300 range—assuming
the transmission is in good condition or rebuilt. I am very impressed with the AX-15’s use of caged needle
roller bearings, heavy roller and ball bearings, and its close, selective fit tolerances. Good gear stamina
is obvious. Unless you have the need for a retrofit and massive NV4500 truck transmission, the AX-15 gearbox
will work well for a long time!

In model year 2000, the TJ Wrangler six-cylinder models switched to
the New Venture 3550 transmission. This is also a rugged, light-duty truck type, comparable to the AX-15. The
NV3500, a similar design, has served in the Dodge Dakota pickup and similar industry applications. Used
behind the 4.0L Jeep engine, the NV3550 will hold up well and also tolerate larger output engine swaps. I
would trust this unit, if in good condition or rebuilt, behind a 250-300 horsepower small-block
V-8.

YJ and TJ Wrangler Manual
Transmission Identification

The manual transmission types used in the YJ and TJ Wrangler are easy
to spot. These illustrations will help sort out the differences between the units.

Illus. 4: At left is the
AX-15 transmission with an internal slave/release bearing front retainer. In the middle is the AX-5 with an iron
front bearing retainer slipped in place to show the difference between internal and external slave designs.
(Note the sleeve where the release bearing rides; an internal slave/bearing retainer would lie in front of the
transmission.)At right is the Peugeot BA 10/5 unit used from 1987-89
in 4.2L six-cylinder models. It, too, has the internal slave/bearing front retainer plate. Note the shape of the
front faces and cases. The AX-15 has an aluminum intermediate plate between the front case and extension/adapter
housing. The AX-5 uses an iron plate here.

Illus. 5: The release bearing assembly, separate from the clutch
housing and front of the transmission, shows the relationship between the pipes, bearing and hydraulic slave.
The assembly is replaced as a rather pricey release bearing/slave unit. Hydraulic pieces are much like a
brake system, with a master cylinder and pipes. Fluid is brake type. I use DOT-4 type (compatible with
factory recommended DOT-3 type) to resist engine and clutch heat. Recommended DOT-3 will
work.

Illus. 6: NV4500 transmission (iron unit at back, rated for 14,500
pound loads) is found in Dodge and GM ¾-ton and 1-ton trucks! Massive, with huge gear sets, these units have
found their way into YJ and TJ Wrangler models as the ultimate torque monster. The later, more common units
boast a 5.61:1 first gear plus a 27% overdrive fifth. At front is the TJ’s NV3550 (without its front/release
bearing retainer).

Note: In myJeep CJ Rebuilder’s Manual: 1972-1986(Bentley Publishers), I detail the NV4500 retrofit
into a 94-inch wheelbase Jeep 4x4 chassis. If you want to perform this swap, refer to that book or an Advance
Adapters catalog covering the Jeep transmission swaps. Advance Adapters provides the conversion pieces—and
even the transmission—for this swap. The AX-15 or NV3550 is ample for the weight, horsepower and requirements
of most YJ or TJ Wranglers. Although the first gear ratio is not as low as the NV4500, the AX-15 or NV3550
provides smoother shifting and closer gear ratio spacing than the heavy-duty NV4500. For better backcountry
crawl ratios, the trend is now toward transfer cases with lower gearing in low range mode, ratios ranging
from the TJ Rubicon’s 4:1 Rock-Trac to Advance Adapters’ Atlas II transfer cases—available in 3.8:1, 4.3:1
and 5.0:1ratio.

Late TJ models receive a light-duty NV1500 transmission coupled to the
‘Power Tech’ 2.4L engine. For model year 2005 and newer, all TJ Wrangler applications use the NSG370
six-speed manual transmission. The NV3550, NV1500 and NSG370 units have good warranty records to date. These
late units, still under long factory warranties at this time, will be available from the Jeep®/Mopar Remanufactured parts program in the years ahead. Service parts will be
available to the aftermarket; however, I would advise your consideration of a Mopar rebuilt/exchange
transmission when working with the NV1500, NV3550 or NSG370 gearboxes.

Spline Count Concerns When Upgrading to the AX-15, NV3550, NV4500 and Other
Transmissions

The AX-5 is a light duty transmission, and I recommend upgrading to
the AX-15 transmission on 2.5L four-cylinder models used for moderate to rugged trail service. This can be
done with the use of a 1990s 2.5L Dodge Dakota four-cylinder clutch housing and clutch linkage
components.

Illus. 7: Be aware that the AX-15 and NV3550 have a 23-spline output.
The AX-5 and Peugeot BA 10/5 units have a 21-spline output. If you upgrade from the AX-4, AX-5 or BA 10/5 to
an AX-15, plan on either 1) swapping the NP/NV231 transfer case into the early YJ four-cylinder model
equipped with the factory NP207 AMC unit, or 2) purchasing a 23-spline transfer case input for the NP207 from
Advance Adapters—while supplies last. (The NP231 swap is my recommendation, a version with provision for
shifting the vacuum disconnect front axle.) Shown here is an AX-15 with nearly ½-inch stick-out length of the
output shaft (beyond the face of the extension housing). You must use a 23-spline transfer case input gear on
the NP231 that matches the transmission’s spline count and also the output stick-out
length.

Note—All NV3550 transmissions use a 23-spline output shaft that ends
flush with the extension/adapter housing.

If you plan more than just routine driving and light trail use with
your 2.5L four, consider the AX-15 swap. I do not include the rebuild of the AX-5 unit in this book, although
the principles involved in AX-15 transmission work will prove helpful. Always use a factory-level service
manual when working with an unfamiliar, technical system like the AX-5 or a BA 10/5
transmission.

The NV3550 is also available for retrofitting in place of the AX-15 in
six-cylinder models, using factory or recycled OEM pieces from an NV3550 or AX-15 equipped model. (All NV3550
transmissions use an external slave cylinder arrangement and a mechanical release arm for the release
bearing.) Keep in mind that the transfer case input and transmission output shaft splines must be compatible
in tooth count and stick-out dimensions. For the ultimate torque and horsepower loads, some turn to the
NV4500 truck transmission swap, a somewhat complex conversion for a 94-inch wheelbase Jeep
4x4.

Note—Advance Adapters can supply the correct NP/NV231 or NP207
transfer case input gear to match a given transmission’s output shaft spline count and length. This will
solve the stick-out length and spline count issues. The NP207 gears will likely be phased out as owners find
it useful to install a more rugged, readily available and affordable NP/NV231 transfer case. Consult the
Advance Adapters catalog for these components or NV4500 transmission swap pieces.